1. Field of the Invention
The present invention relates to a wireless sensor network (WSN), and more particularly, to a wireless sensor network having a hierarchical structure guaranteeing end-to-end delay considering scalability of a network, and a routing method thereof.
2. Description of the Related Art
A WSN generally includes a large scale amount of sensor nodes which are densely distributed in a sensor field. In a large scale sensor network, a delay in data delivery may cause a critical result to an application sensitive to the such delays. In actual real-time services such as a military application, a target must be quickly recognized and information on the target must be delivered to a command post located at a remote place.
In order to embody real-time services such as in a battle field application, not only extensibility of a large scale network but also a delay guarantee of service must be taken into consideration. One method of minimizing delay uses a hierarchical structure and a short hop count. However, even when the hop count is short, if the quality of a link is not good, the delay may be further extended due to continuous retransmission. When the quality and reliability of the hierarchical structure and link are guaranteed, a delay guarantee of real-time service can be processed.
In the existing routing methods, Mint routing measures the quality of a link using a routing metric. The Mint routing uses an exponentially weighted moving average (EWMA) method of calculating values of a newly expected link quality and a previously expected link quality using a weight control. However, since the Mint routing uses a nonlinear link expectation, it is not sufficient to determine a routing path having superior link quality information along the path, so meaning that there is a problem in routing stability. The determination of the route for the next hop is made according to the aggregate of quality of the links flowing along different paths. That is, the determination of the route for the next hop signifies the route on the path for the next hop selection, not just the link of the next hop.
As described above, it is problematic to use the Mint routing metric for the real-time application because it cannot sufficiently reflect the quality and status of the current link. Although there are lots of lossy links in view of the overall sensor network, a loss rate dynamically varies according to environmental elements. The link-level retransmission is unreliable and each hop requires one or more retransmissions to compensate for a lossy channel. This may lead to data delivery delay in the real-time service.